In the meantime, a facility that protects Mars rocks from terrestrial contamination and safeguards Earth organisms from possible extraterrestrial microbes will take seven years or longer to design, construct and test, the National Research Council scientists said in a report released Tuesday.

"Building this type of quarantine facility is a project of enormous complexity," said John Wood, a Harvard University astrophysicist and lead author of the report, in a statement. "We strongly recommend that this process get under way as soon as possible."

A daunting challenge in building such a laboratory is the unparalleled need for both biological containment, to protect Earth from possible contamination, and clean-room conditions, to prevent terrestrial microbes from infiltrating the samples.

Biological containment centers, such as those housing samples of dangerous pathogens that spread quickly, have rooms with artificially low air pressure, which prevents leaking air from escaping into the environment.

Scientific clean rooms, however, have chambers with air pressure higher than normal, which ensures that external substances do not contaminate enclosed samples.

A facility with both capabilities does not exist. And the NRC cautions that extensive testing must be taken before one can be built. The council report recommended that NASA manage such a laboratory, and that it be located next to an existing complex with the highest biological containment capability, a so-called "level-four" facility.

Candidates include those operated by the Centers for Disease Control and Prevention in Atlanta, the U.S. Army Medical Research Institute for Infectious Diseases in Fort Detrick, Maryland, and a proposed medical containment center at the University of Texas at Galveston.

The report recommends that only rudimentary biological tests take place in the quarantine facility. Should the samples show no signs of life, they can be sent to other laboratories for further study. If there are mixed signs, the samples should be sterilized and then distributed to other labs.

If testing reveals unmistakable evidence of life, the samples should not be released until a panel of biologists considers the predicament, according to the NRC, an affiliate of the National Academies of Sciences and Engineering.

Not everyone is convinced such measures would ensure the safety of the terrestrial biosphere should the Mars samples possess life.

A canister returning with the Mars sample is expected to crash land into Utah desert. And Mars researcher Barry E. DiGregorio warns that it could be contaminated on the outside. He cites the example of the Mars Pathfinder lander, which scientists estimate was covered with surviving terrestrial spores when it landed on the red planet.

"We can't guarantee that a spacecraft going to Mars is sterile. How can we guarantee that anything returning is?" said DiGregorio, founder of the International Committee Against Mars Sample Return.

The committee includes among its members Gilbert Levin, a scientist who designed and managed an experiment on a NASA lander that tested for life on Mars in the 1970s.

While the space agency concluded otherwise, Levin and a colleague determined that his tests offered strong evidence of biological forces at work in the martian soil.

Extensive testing should take place by robots on Mars or by scientists in Earth orbit before samples are brought down to our planet, DiGregorio recommended.